1. Field of the Invention
The present invention relates to an electrically powered foldable rearview mirror for automobiles and, more particularly, to a control device for an electrically powered foldable rearview mirror for automobiles which is provided with a simple and compact circuit resulting in a safer operation.
2. Description of the Relevant Art
The known electrically powered foldable rearview mirror for automobiles projects from a vehicle body side and is folded when the automobile is parked. For example, as disclosed by Japanese Utility Model Registration Application Publication Sho 63-169341, the electrically foldable rearview mirror comprises a mirror body, a motor attached to a frame for folding the mirror body electrically, a shaft erected on a base where the mirror body is attached, a gear engageably and removably mounted on the shaft and a speed reduction mechanism provided between the motor and the gear. The gear is arranged around the shaft and has a clutch mechanism so that, when the mirror body is folded electrically, the gear is fixed on the shaft, but when the mirror body is forced to rotate, the gear is free of the shaft. An end gear of the speed reduction mechanism engages with a clutch gear. Both the speed reduction mechanism containing the end gear and the frame for supporting that mechanism are centered around the shaft and normally and reversely rotated around the shaft by an actuation of the motor, so that the mirror body reaches a normal (erected) state or a folded state.
The rotation mechanism for the mirror body includes a mirror body positioning means (location means) comprising a stopper plate and steel balls and is provided between a shaft root portion which is erected on an extension of a base and a frame root portion.
Steel balls are supported on a concave spherical seat formed on an undersurface of the frame root portion so as to rotate with the frame when the mirror body is rotated. When the mirror body reaches the erected state, the steel balls contact with a stopper shoulder of a stopper plate.
Rotation stop means for the mirror body comprises a convex portion and an arc-shaped groove provided under the frame root portion to receive the convex portion. When the frame rotates around the shaft, the arc-shaped groove of the frame root portion is guided over the convex portion. When the mirror body is held in the folded state, the convex portion which is part of the rotation stop means for the mirror body in a folded state contacts with one end of the arc-shaped groove to stop the mirror body rotation.
In a conventional method of switching off a motor driving circuit to stop the mirror body rotation in the erected state or the folded state when the electrically foldable rearview mirror is erected or folded, for example, a mechanism is provided including a limit switch, a plate contact switch or the like. As disclosed by Japanese Utility Model Registration Application Publication No. Hei 4-76196, there is another method which is based on a characteristic of a PTC component (element), and in which the motor driving circuit is cut off by increased electric current when the motor rotation is locked. Furthermore it is known to control the motor by a sensor including for example a photoreflector or a magnetic induction component (element) to detect the motor rotation.
However in a conventional method of switching off the motor driving circuit as described above such as that using above-mentioned mechanical mechanism, it is difficult to adjust the circuit switching position at the mirror body erected state or the folded state and a wrong operation such as an imperfect contact is easily caused by foreign contamination in the rotating contact portion. Moreover in the method using the PTC component, during a comparatively large time lag from the occurrence of the overload on the motor to a predetermined temperature of PTC component, the electric power is still being supplied to the motor. Consequently there is a danger that the positioning accuracy for the mirror body is so decreased that the mirror body moves out of the controlled state and the motor is damaged by overheating. The durability of parts is decreased because the driving circuit is loaded or because of heat generated by continuous excess current in the circuit and the circuit parts are damaged or ignite.
Moreover in the method based on detecting the motor rotation, there is danger that the sticking dust, contaminants or the like on the sensor portion cause faulty operation of the device. Furthermore due to a complicated electronic circuit composed of an amplifier circuit, a driving circuit or the like which is necessary for converting a sensor signal to the motor control signal, the system becomes comparatively expensive.